The $conic input group
This group controls the detailed behavior of the
$statpt method=CONIC Lagrange multiplier based CIs/ISCs optimizer
called CONIC. In addition to the parameters described below, CONIC is
also controlled by the parameters of the $statpt group.
shift The final target value of
Δ = Ej - Ei The default value is 0.0001 Hartree. For true CIs, it is
almost impossible to reliably converge calculations to values of Δ
less than ca. 0.0001 Hartree. For ISCs, it is possible to use tighter
convergence criteria, up to shift=1.0d-7 or 1.0d-8. The difference in
convergence behavior between true CIs and ISCs is related to non-zero
non-adiabatic coupling between quasi-degenerate states in the case of
CIs and the absence of this coupling in the case of ISCs.
shift0 The intermediate target value
of Δ = Ej - Ei The default value is 0.0001 Hartree. For CIs, the
recommended value is 0.001 Hartree. For ISCs, the recommendation is to
always set shift0 to the same value as shift. The CONIC optimizer
first tries to converge CIs/ISCs to Δ values equal to or below of
shift0 using loosened values of various convergence thresholds.
During this process (which is internally called "phase 1"), the CONIC
optimizer gathers information on the behavior of the particular system
of interest. Upon achieving the initial convergence, the optimizer
switches to the original values of various convergence thresholds and
continue optimization trying to achieve tighter convergence with the
value of Δ equal to or below of shift. This is internally called
"phase 2". Here, the previously gathered information is used to
achieve the final convergence. In particular, the first few
optimization steps of phase 2 attempt to rapidly decrease Δ using
directed steps along the "most significant" modes while the rest of
the steps take care of the final relaxation of the "less significant"
modes.
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